1. Field of the Invention
The present invention relates to a driving device for an ultrasonic motor which drives a rotor of the motor using a travelling vibration wave generated in an elastic member by means of a piezoelectric member.
2. Related Background Art
In ultrasonic motors which utilize a travelling vibration wave, a piezoelectric member is vibrated by the application of an alternating drive voltage, and a travelling vibration wave is thereby generated in an elastic member adhered to the piezoelectric member to frictionally drive a rotor which is in contact with the elastic member. Such ultrasonic motors have been disclosed in, for example, Japanese Patent Laid-Open No. 59-111609.
FIG. 2 is a schematic cross-sectional view of such an ultrasonic motor, and FIG. 3 is a plan view of an ultrasonic motor as seen when looking from the direction of a piezoelectric member.
In the ultrasonic motor shown in FIGS. 2 and 3. a piezoelectric member 8-2 is adhered to one surface of an elastic member 8-1. The elastic member 8-1 and the piezoelectric member 8-2 in combination constitute a vibrating member 8-3. A rotating member 8-5 is in contact with the other surface of the elastic member 8-1 with a slider 8-4 therebetween. The slider 8-4 and the rotating member 8-5 in combination form a rotor 8-6.
As shown in FIG. 3, the piezoelectric member 8-2 has on its surface four electrodes 8-2a, 8-2b, 82c and 8-2d. .pi./2 out-of-phase alternating drive voltages are applied to the drive electrodes 8-2a and 8-2b. The electrode 8-2c is grounded. The electrode 8-2d is used to take out an alternating output voltage which corresponds to the vibrations of the vibrating member 8-3.
Drive controlling devices for such an ultrasonic motor have been proposed in, for example, U.S. Pat. No. 4,510,411 and Japanese Patent Laid-Open No. 61-251490. These drive controlling devices are designed to control the frequency of an alternating drive voltage signal (1) using a voltage taken out from the monitor electrode 8-2d or (2) using a phase difference between an alternating voltage signal applied to the piezoelectric member and a voltage signal output from the monitor electrode.
However, the system designed to control the frequency of the alternating drive voltage signal using the alternating voltage signal output from the monitor electrode has the following drawbacks.
(1) Since the area of the monitor electrode 8-2d is in general smaller than that of the driving electrode 8-2a or 8-2b, it has a high impedance, and is thus readily affected by a circuit connected thereto.
(2) The output of the monitor electrode 8-2d varies when the drive direction is altered due to a shift in the positional relation between the driving electrodes 8-2a and 8-2b and the monitor electrode 8-2d or because of errors in the area of the monitor electrode 8-2d.
(3) As the frequency of the alternating drive voltage signal applied to the drive electrodes 8-2a and 8-2b deviates from the resonant frequency inherent to the ultrasonic motor, the amplitude of the travelling vibration wave decreases, dropping the voltage output from the monitor electrode 8-2d. This makes detection of the output of the monitor electrode 8-2d impossible on occasions when the frequency of the alternating drive voltage signal is controlled to a value which greatly deviates from the resonant frequency so as to reduce the speed. In that case, distortion occurs in the waveform of the output of the monitor electrode 8-2d. This makes measurement of the phase difference between the alternating drive voltage signal and the output of the monitor electrode 8-2d difficult.
(4) The problem labelled (3) occurs again when the alternating drive voltage is dropped to reduce the speed of the motor.
(5) Since the phase difference between the rectangular wave of the alternating drive voltage and the waveform of the output of the monitor electrode 8-2d shifts because of reversal of the direction of the motor rotation, a circuit for shifting the phase of the alternating drive voltage for each reversal is required.